Children's participation in medical research has always presented special challenges. What is their level of understanding of the disease process and the treatment process? How is their understanding affected by their anxiety associated with hospitalization? How should their assent be interpreted in light of their various levels of understanding? This study focuses on a part of these interlocking issues (a) by comparing levels of understanding of 3 age groups (7-12, 13-18, 19-30), (b) by examining level of understanding of the disease and treatment processes in relation to standard measures of cognitive functioning - unrelated to disease, and (c)\by assessing level of anxiety. These measures permit exploration of developmental differences in comprehension of one's medical situation, and allow examination of the effects of anxiety on comprehension at each of the age levels. A more general question, the effects of stress and anxiety on cognitive functioning, is addressed in the comparisons of the patient's level of cognitive functioning on the non-stressful content of the standard cognitive tests with his/her comprehension and reasoning on the stressful medical content. Participants are child, adolescent, and adult inpatients and a comparison group of healthy participants. Psychological assessments consist of standard cognitive tests, a test of anxiety level, and interviews. Preliminary findings show that age predicts participants' level of reasoning concerning stressful content related to their disease and treatment, but verbal abilities (Peabody, Picture Vocabulary Test) predict level of reasoning on a standardized test of knowledge of illness. Anxiety was associated with lower levels of reasoning on the non-stressful content.